1. Field of the Invention
The present invention relates to a robot device having a control unit to control a pair of actuators for rotational driving of a link, a robot control method, a program, and a recording medium.
2. Description of the Related Art
The ability for end effectors to flexibly come into contact with objects is becoming important in control methods of manipulators. Application of this to industrial robots could realize cooperative work between humans and robots, facilitating work of fitting parts to each other by controlling the direction of flexibility of end effectors, and so forth.
Also, applying this to legged locomotive robots could alleviate shock to the torso by coming into contact with the ground softly, and enable smooth ambulation over unsmooth terrain by absorbing difference in height.
In order to realize control of flexibility of the end effectors, impedance control where force sensors are worn on the end effectors, control using artificial muscle actuators, and so forth, is being performed. It is known that while human muscles are actuators, they are also control mechanisms with variable viscoelasticity. Rubber pneumatic artificial muscle actuators, of which the McKibben type artificial muscle actuator is representative, are similar to human muscles with regard to their viscoelastic properties. Now, controlling the softness of the artificial muscle actuators included in a manipulator enables contact with an object with optional end effector flexibility.
“Mechanical Properties of Robot Arm Operated with Muscle Coordinate System Consisted of Bi-articular Muscles and Mono-articular Muscles”, Toru OSHIMA, Tomohiko FUJIKAWA, and Minayori KUMAMOTO, Journal of The Japan Society for Precision Engineering, Vol. 66, No. 1 pp. 141-146 (hereinafter “Oshima et al”) proposes a “three-pair six-muscle model” manipulator having, in addition to artificial muscle actuators which drive a first link and a second link, a bi-articular simultaneous driving actuator to drive the first link and second link at the same time. Oshima et al have verified that by making the elasticity of the artificial muscle actuators to be equal, in a case where external force is applied to the end effectors along a line connecting a first joint and a end effector, there is exhibited a feature in stiffness characteristics in that the eternal force direction of the end effector and the direction of motion agree. McKibben actuators are difficult to use in motion control, due to the following reasons. The viscoelastic properties of McKibben actuators are nonlinear, and also control has to be applied to an antagonistic arrangement.
On the other hand, Japanese Patent Laid-Open No. 2012-86354 discloses deriving a simple feedback control system using saturation of control input, which controls joint angle at the same time as with stiffness control of the end effectors. However, McKibben type artificial muscle actuators have to control pneumatic pressure by opening and closing valves, and it is known to be difficult to obtain high responsivity. Accordingly, research is being advanced on variable stiffness actuation (VSA) actuators, which use motors as force generating elements, and further combine mechanical elements such as springs and dampers, to enable variable viscoelasticity like that of an artificial muscle actuator to be realized. VSA also has a feature that the output direction of force is not restricted to the direction of contraction.
“Bidirectional Antagonistic Variable Stiffness Actuation: Analysis, Design & Implementation” Florian Petit, Maxime Chalon, Werner Friedl, Markus Grebenstein, Alin Albu-Schaffer and Gerd Hirzinger, 2010 IEEE International Conference on Robotics and Automation, pp. 4189-4196 (hereinafter “Petit et al”) describes configuration a linear type VSA by linearly joining motors and non-linear springs, and situating linear type VSAs in an antagonistic manner as to a link. Both motors are used to control reserve tensile force of the non-linear springs, thereby changing the angle and joint stiffness of the manipulator. Petit et al describes providing non-linear springs with initial deformation, and perpetually performing control for antagonistic driving, which is referred to as “normal mode”. In a case where a greater joint driving torque becomes necessary, such as external force being applied to the end effector or the like, the reserve tensile force is reduced. This releases the antagonistic driving state, so both VSAs apply torque in the same direction to the joint, which is referred to as “helping mode”.
Petit et al describes antagonistic placement of linear type VSAs to perform antagonistic driving when accurate joint stiffness is necessary, and to perform non-antagonistic driving when greater torque is necessary. On the other hand, the artificial muscle actuators described in Oshima et al and Japanese Patent Laid-Open No. 2012-86354 are modeled with the force generating element, spring, and damper arrayed in parallel. Hereinafter, this arrangement will be referred to as “parallel type VSA”.
Using such a parallel type VSA enables generated force of the force generating element to be applied to the manipulator without passing through the spring and damper, and thus is advantageous in that highly-precise positioning control can be had. However, there has not been proposed as of yet a control system which transitions seamlessly between antagonistic driving and non-antagonistic driving, using a parallel type VSA.